The c-Myc proto-oncogene is required for cell growth and proliferation in all tissues and is structurally altered by a variety of mechanisms (amplification, translocation, promoter insertion) in many tumor types. In breast carcinomas, amplification of the c-Myc locus is detectable in 15% of cases, and the expression of c-Myc is apparently deregulated in the majority of cases. The general goal of this project is to understand the function of the c-Myc protein and its role in breast carcinogenesis. In particular, we will: 1) Identify of the proteins interacting with c-MYC in vivo. Although a number of proteins binding c-Myc have been identified, their physiological role remains unclear. We propose to: i) isolate the protein complex containing c-Myc in breast epithelial cells; ii), identify its components; and iii) determine their functional role (selected proteins). 2) Identify the role of MXI in the c-MYC autoregulatory circuit and its inactivation in breast carcinoma. c-MYC expression is modulated by a negative autoregulatory circuit that is blocked in transformed cells (including breast carcinoma) leading to c-MYC overexpression. Preliminary data indicate that in this circuit c-Myc directly activates the transcription of Mxi, which, in turn, downregulates c-MYC. Intriguingly, MXI is involved in chromosome 10q23 deletion in breast carcinoma suggesting that it could function as a tumor suppressor gene whose loss or inactivation could lead to c-MYC overexpression. Based on these findings, we plan to investigate: i) whether MXI can directly bind to the c-MYC promoter region in vivo; ii) whether c- MYC and MXI oppositely regulate the transcription of the same set of target genes, and iii) whether MXI is deleted, mutated or silenced in human breast carcinoma. 3) Examine the role of the c-Myc target genes PKA-Cbeta and telomerase (TERT) in breast development and tumorigenesis. We have identified PKA-Cbeta, the catalytic subunit of PKA, and TERT, the catalytic subunit of telomerase as direct targets of c-Myc, and shown that the induction of PKA-Cbeta is necessary for c-Myc mediated transformation. We will study the role of these genes in breast development and carcinogenesis by constructing mice that constitutively express PKA-Cbeta beta or TERT in the breast epithelium.